The present invention relates to semiconductor devices in which a conductive silicon carbide (SiC) substrate is used in conjunction with Group III nitride active layers. Relevant devices can include light emitting diodes (LEDs) and other devices. In operation, these devices are characterized (in one respect) by the flow of current from a silicon carbide substrate to one or more Group III nitride layers.
As used herein, the term “Group III nitride” refers to those binary, ternary and quaternary compounds formed from the Group III elements and nitrogen. Examples include gallium nitride (GaN), aluminum gallium nitride (AlGaN), and indium gallium aluminum nitride (InGaAlN). In the ternary and quaternary compounds, the atomic fraction of all of the Group III elements taken together is in one-to-one ratio with the nitrogen. Thus (for example), AlGaN is often expressed by the formula AlxGa1-xN, where 0<x<1.
In such structures and devices, the Group III nitride is usually present as one or more epitaxial layers. Group III nitrides provide a wide bandgap and a direct transition between the valence band and the conduction band. The wide bandgap offers a number of electronic advantages such as the capability of emitting photons in the green, blue, violet and ultraviolet portions of the electromagnetic spectrum. The direct transition offers efficiency in such emissions because all of the energy is generated as light.
Silicon carbide offers several advantages as the substrate for such Group III nitride layers. In addition to silicon carbide's physical and electronic advantages (wide bandgap, radiation hardness, high thermal conductivity, stability at high temperatures), it also offers an acceptable crystal lattice match with the Group III nitrides, it can be conductively doped, and it can be grown in transparent crystals that have a high refractive index.
When, however, different semiconductor materials such as SiC and GaN are placed adjacent one another, the difference between the respective conduction band edges encourages the carriers in each adjacent layer to find an equilibrium of the lowest available potential energy. When Group III nitrides are placed adjacent silicon carbide, this creates a relatively large energy barrier for electrons flowing from the silicon carbide substrate towards and into the Group III nitride layer. The presence of this barrier increases the forward voltage of the device thus reducing its efficiency as compared to theoretical maximums and creating heat rather than light or other desired output characteristics.
Accordingly, a motivation exists to attempt to reduce the energy barrier to correspondingly reduce forward voltage and increase the efficiency of any device incorporating this type of structure.
Commonly assigned and co-pending U.S. Patent Application Publication No. 20050158892 (and related applications) discloses a method of improving the voltage characteristics of a Group III nitride-silicon carbide interface by implanting the SiC with dopants and then annealing the resulting structure. The higher doping concentration in the silicon carbide helps reduce the energy barrier and thus reduce the forward current.
Although this technique offers certain advantages, the implanting and annealing steps add complexity to the process and require additional manufacturing time and equipment. As in any manufacturing or other sequential process, if steps can be eliminated, the overall efficiency of the process usually can be increased.
Furthermore, when implanting silicon carbide with a relatively high carrier concentration in order to lower the energy barrier, the higher doping can result in greater damage to the silicon carbide crystal. This can in turn cause undesired optical absorption in LED structures, can degrade the electronic properties of the SiC, and can create surface defects that can affect the quality of later epitaxial growth.